Extracellular signal-regulated kinase activation differentially regulates platelet-derived growth factor's actions in hepatic stellate cells, and is induced by in vivo liver injury in the rat

Inhibition of PDGF, TGF-β, and Abl signaling and reduction of liver fibrosis by the small molecule Bcr-Abl tyrosine kinase antagonist Nilotinib

BACKGROUND & AIMS

Nilotinib is a novel tyrosine kinase inhibitor of Bcr-Abl and other kinases. In this study, we have examined its activity as an anti-fibrotic agent.

METHODS

The in vitro effect of Nilotinib on rat and human HSCs was assessed using proliferation assays and Western blotting. The in vivo antifibrotic efficacy of Nilotinib was assessed in mice with liver fibrosis induced by CCl(4) and bile duct ligation (BDL).

RESULTS

Nilotinib inhibited proliferation, migration, and actin filament formation, as well as the expression of α-SMA and collagen in activated HSCs. Nilotinib induced apoptosis of HSCs, which was correlated with reduced bcl-2 expression, increased p53 expression, cleavage of PARP, as well as increased expression of PPARγ and TRAIL-R. Nilotinib also induced cell cycle arrest, accompanied by increased expression of p27 and downregulation of cyclin D1. Interestingly, Nilotinib not only inhibited activation of PDGFR, but also TGFRII through Src. Nilotinib significantly inhibited PDGF and TGFβ-simulated phosphorylation of ERK and Akt. Furthermore, PDGF- and TGFβ-activated phosphorylated form(s) of Abl in human HSCs were inhibited by Nilotinib. In vivo, Nilotinib reduced collagen deposition and α-SMA expression in CCl(4) and BDL-induced fibrosis. These beneficial effects were associated with suppressed expression of procollagen-(I), TIMP-1, CD31, CD34, VEGF, and VEGFR. Nilotinib could induce HSC undergoing apoptosis in vivo, which was correlated with downregulation of bcl-2. We also observed reduced expression of phosphorylated ERK, Akt, and Abl in the Nilotinib-treated CCl(4) and BDL livers. In addition to its antifibrotic activity, the drug was hepatoprotective and reduced the elevations of ALT and AST after CCl(4) and BDL.

CONCLUSIONS

These studies uncover a novel role of Bcr-Abl activity in treatment of liver fibrosis through multiple mechanisms and indicate that Nilotinib represents a potentially effective antifibrotic agent.

Intracellular reactive oxygen species are required for directional migration of resident and bone marrow-derived hepatic pro-fibrogenic cells

BACKGROUND & AIMS

Liver fibrogenesis is sustained by myofibroblast-like cells originating from hepatic stellate cells (HSC/MFs), portal fibroblasts or bone marrow-derived cells, including mesenchymal stem cells (MSCs). Herein, we investigated the mechanistic role of intracellular generation of reactive oxygen species (ROS) and redox-sensitive signal transduction pathways in mediating chemotaxis, a critical profibrogenic response for human HSC/MFs and for MSC potentially engrafting chronically injured liver.

METHODS

Intracellular generation of ROS and signal transduction pathways were evaluated by integrating morphological and molecular biology techniques. Chemokinesis and chemotaxis were evaluated by wound healing assay and modified Boyden's chamber assay, respectively. Additional in vivo evidence was obtained in human specimens from HCV-related cirrhosis.

RESULTS

Human MSCs and HSC/MFs migrate in response to a panel of polypeptide chemoattractants and extracellularly generated superoxide anion. All polypeptides induced a NADPH-oxidase-dependent intracellular rise in ROS, resulting in activation of ERK1/2 and JNK1/2. Moreover, menadione or 2,3-dimethoxy-1,4-naphthoquinone, which generate intracellular superoxide anion or hydrogen peroxide, respectively, induced ERK1/2 and JNK1/2 activation and migration. JNK1 activation was predominant for migration as shown by specific silencing. Finally, activation of ERK1/2 and JNK1/2 was found in extracts obtained from HSC/MFs during the course of an oxidative stress-mediated model of liver injury and phosphorylated JNK1/2 isoforms were detected in α-smooth muscle actin-positive myofibroblasts lining fibrotic septa in human cirrhotic livers.

CONCLUSIONS

Intracellular generation of ROS, through activation of specific signaling pathways, is a critical event for directional migration of HSC/MFs and MSCs.

Fibrogenesis in nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis includes a wide spectrum of liver injury, ranging from simple inflammation to fibrosis and cirrhosis. Whereas simple steatosis has a benign clinical course, steatohepatitis is a recognized cause of progressive liver fibrosis and can develop, in some circumstances, into cirrhosis. The main cause of fibrogenesis is represented by the activation of myofibroblastic cells, which then start to produce matrix filaments. Matrix-producing cells, although mainly constituted of hepatic stellate cells, may have a different origin in the liver. This article will provide information on the sources of matrix-producing cells and the mechanisms involved in the development of fibrogenesis, with particular attention paid to the pathophysiological implications leading from steatohepatitis to fibrosis and cirrhosis.

Silencing tissue inhibitors of metalloproteinases (TIMPs) with short interfering RNA reveals a role for TIMP-1 in hepatic stellate cell proliferation

Myofibroblastic, activated hepatic stellate cells (HSC) play a pivotal role in the development of liver fibrosis through the secretion of fibrillar collagens and the tissue inhibitors of metalloproteinase (TIMP)-1 and -2. TIMPs are believed to promote hepatic fibrosis by inhibiting both matrix degradation and apoptosis of HSC. In other cell types, there is evidence that TIMP-1 has effects on proliferation, however the role of TIMPs in the regulation of HSC proliferation remains unexplored. Therefore, we have used short interfering RNA (siRNA) to investigate the effects of autocrine TIMP-1 and -2 on HSC proliferation. TIMP-1 and -2 siRNA were highly effective, producing peak target protein knockdown compared to negative control siRNA of 92% and 63%, respectively. Specific silencing of TIMP-1, using siRNA, significantly reduced HSC proliferation. TIMP-1 was localised in part to the HSC nucleus and TIMP-1 siRNA resulted in loss of both cytoplasmic and nuclear TIMP-1. Attenuated proliferation was associated with reduced Akt phosphorylation and was partially rescued by addition of recombinant TIMP-1. We have revealed a novel autocrine mitogenic effect of TIMP-1 on HSC, which may involve Akt-dependent and specific nuclear mechanisms of action. We suggest that TIMP-1 might promote liver fibrosis by means other than its previously described anti-apoptotic effect on HSC. Moreover, these findings, together with our previous reports and the emerging data from in vivo studies of TIMP inhibition, provide strong evidence that TIMP-1 is mechanistically central to liver fibrosis and an important potential therapeutic target.

Anthocyanins isolated from the purple-fleshed sweet potato attenuate the proliferation of hepatic stellate cells by blocking the PDGF receptor

During the process of liver fibrosis, hepatic stellate cells (HSCs) play a critical role in the increased formation and reduced degradation of extracellular matrix in the liver. We investigated the anti-proliferative effects of an anthocyanin fraction (AF), isolated from the purple-fleshed sweet potato, on platelet-derived growth factor (PDGF)-BB-dependent signaling pathways in HSC-T6 cells. HSC proliferation plays a pivotal role in liver fibrogenesis. The AF suppressed HSC activation, including PDGF-induced proliferation and α-smooth muscle actin (α-SMA) expression. Additionally, AF inhibited PDGF-BB-induced Akt and ERK1/2 phosphorylation. AF inhibited the phosphorylation level of PDGF receptor-β (PDGFR-β) following PDGF-BB stimulation, providing a mechanism for the inhibition of AF-mediated kinase. These results suggest that AF suppresses HSC proliferation by blocking PDGFR-β signaling, inhibiting Akt and ERK1/2 activation and α-SMA expression.

Peptidyl arginine deiminase: A novel immunohistochemical marker for liver fibrosis in patients with chronic hepatitis

Peptidylarginine deiminase (PAD) is an enzyme known to be involved in the pathogenesis of rheumatoid arthritis (RA). Since many of the molecular events present in the joints in RA also take place in the injured liver, we postulated in this study that PAD may be involved in liver fibrosis. The objectives of this study therefore were to find out if PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and if it is associated with METAVIR activity and fibrosis scores. Liver biopsies were obtained from 100 patients with chronic liver diseases between September 2006 and 2007. The biopsies were scored by two histopathologists according to the METAVIR activity and fibrosis scores after histological preparation. Immunohistochemistry for PAD was performed on the biopsies using a monoclonal antibody against PAD. PAD could not be demonstrated in normal liver biopsies but was found in the hepatocytes of patients with chronic hepatitis. PAD labeling could distinguish patients with no fibrosis from either F1 or F2 or F3 or F4 fibrosis. Similarly, PAD labeling could separate patients with no inflammatory activity from those with mild or moderate or severe activity. We concluded that PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and that its immunodetection was significantly associated with Metavir activity and fibrosis scores.

Triplex forming oligonucleotides against type α1(I) collagen attenuates liver fibrosis induced by bile duct ligation

Liver fibrosis is a consequence of chronic liver disorders which lead to the accumulation of extracellular matrix (ECM). Particularly, there is an increased accumulation of collagen in the fibrotic liver. We have therefore used a triplex forming oligonucleotide (TFO) against the type α1(I) collagen and evaluated, whether it can attenuate liver fibrosis induced by common bile duct ligation (CBDL) in rats. There was a significant decrease in hydroxyproline levels and Masson's trichrome staining for collagen in TFO-treated CBDL groups compared to non-treated CBDL group. There was over expression of type α1(I) collagen, α-smooth muscle actin (α-SMA) and TGF-β1 expression in the CBDL group compared to TFO-treated CBDL group. Also, the serum alanine transaminase (ALT) and aspartate transaminase (AST) concentrations were less in the TFO treated group compared to non-treated CBDL group. There was also less neutrophils accumulation in TFO treated CBDL group assayed by myeloperoxidase (MPO) assay. These results suggests that TFO can be used to downregulate type 1 collagen gene expression and can alleviate liver fibrosis induced by common bile duct ligation.

New insights into the antifibrotic effects of sorafenib on hepatic stellate cells and liver fibrosis

BACKGROUND & AIMS

During the process of liver fibrosis, hepatic stellate cells (HSCs) play a critical role in the excessive production of extracellular matrix (ECM). We evaluated the therapeutic effects of sorafenib, a multiple receptor tyrosine kinase inhibitor, targeting platelet-derived growth factor (PDGF) receptor and the Raf/extracellular-signal-regulated kinase (ERK) signaling pathway, on liver fibrosis and HSC proliferation.

METHODS

The in vivo effects of sorafenib were monitored in the livers of rats with liver fibrosis, and simultaneously proliferation assays, apoptosis induction studies, and collagen synthesis measurement were conducted in vitro in rat and human HSCs and primary HSCs.

RESULTS

Sorafenib treatment attenuated liver fibrosis and was associated with a significant decrease in intrahepatic fibrogenesis, hydroxyproline accumulation and collagen deposition. Sorafenib reduced HSC proliferation and resulted in significantly higher levels of apoptosis. Moreover, sorafenib downregulated Cyclin D1 and Cyclin-dependent kinase 4 (Cdk-4), simultaneously increased expression of Fas, Fas-L, and Caspase-3, and decreased the ratio of Bcl-2 to Bax. Sorafenib treatment increased the ratio of matrix metalloproteinases (MMPs) to tissue inhibitor of matrix metalloproteinases (TIMPs) and reduced collagen synthesis in HSCs. Sorafenib inhibited the phosphorylation of ERK, Akt and 70-kDa ribosomal S6 kinase (p70S6K), both in vitro and in vivo.

CONCLUSIONS

Sorafenib induces the suppression of collagen accumulation and HSC growth warranting the use of sorafenib as a potential therapeutic agent in the treatment of liver fibrosis.

Cigarette smoking exacerbates nonalcoholic fatty liver disease in obese rats

The prevalence of cigarette smoking (CS) is increased among obese subjects, who are susceptible to develop nonalcoholic fatty liver disease (NAFLD). We investigated the hepatic effects of CS in control and obese rats. Control and obese Zucker rats were divided into smokers and nonsmokers (n = 12 per group). Smoker rats were exposed to 2 cigarettes/day, 5 days/week for 4 weeks. The effects of CS were assessed by biochemical analysis, hepatic histological examination, immunohistochemistry, and gene expression analysis. Phosphorylation of AKT and extracellular signal-regulated kinase (ERK) and quantification of carbonylated proteins were assessed by western blotting. As expected, obese rats showed hypercholesterolemia, insulin resistance, and histological features of NAFLD. Smoking did not modify the lipidic or glucidic serum profiles. Smoking increased alanine aminotransferase serum levels and the degree of liver injury in obese rats, whereas it only induced minor changes in control rats. Importantly, CS increased the histological severity of NAFLD in obese rats. We also explored the potential mechanisms involved in the deleterious effects of CS. Smoking increased the degree of oxidative stress and hepatocellular apoptosis in obese rats, but not in controls. Similarly, smoking increased the hepatic expression of tissue inhibitor of metalloproteinase-1 and procollagen-alpha2(I) in obese rats, but not in controls. Finally, smoking regulated ERK and AKT phosphorylation. The deleterious effects of CS were not observed after a short exposure (5 days).

CONCLUSION

CS causes oxidative stress and worsens the severity of NAFLD in obese rats. Further studies should assess whether this finding also occurs in patients with obesity and NAFLD.

Plasma osteopontin concentration correlates with the severity of hepatic fibrosis and inflammation in HCV-infected subjects

BACKGROUND

The association between OPN level and the histological severity of hepatic fibrosis and inflammation in hepatitis C virus (HCV) induced liver fibrosis remains unknown.

METHODS

120 chronic HCV-infected subjects and 75 controls were enrolled in this study. Assessment of liver histology was performed based on liver biopsy. Plasma OPN levels were determined.

RESULTS

Significant differences were noted in the mean plasma OPN levels between subjects with extensive fibrosis and those with mild fibrosis (4.29+/-1.01 ng/ml vs. 2.15+/-0.63 ng/ml, respectively; p<0.001). Similarly, the subjects with higher histological activity index (HAI) score had elevated OPN levels than those with mild HAI score (4.41+/-1.11 ng/ml vs. 2.25+/-0.94 ng/ml, respectively; p<0.001). The correlation between the plasma OPN levels and the severity of liver fibrosis degree and HAI score were noted (r=0.945, and r=0.788, respectively both p<0.001). Logistic regression analysis showed that serum OPN was an independent risk factor contributing to extensive liver fibrosis and inflammation (p=0.0018 and p<0.001, respectively) in patients with HCV subjects.

CONCLUSION

The plasma OPN level is correlated with the severity of liver fibrosis and inflammation, suggesting OPN could be used as a biomarker to evaluate the severity of liver damages in HCV subjects.

The coagulation factor Xa/protease activated receptor-2 axis in the progression of liver fibrosis: a multifaceted paradigm

Hepatic fibrosis is a common response to virtually all forms of chronic liver injury independent of the etiologic agent. Despite the relatively large population of patients suffering from hepatic fibrosis and cirrhosis, no efficient and well-tolerated drugs are available for the treatment of this disorder. The lack of efficient treatment options is at least partly because the underlying cellular mechanisms leading to hepatic fibrosis are only partly understood. It is thus of pivotal importance to better understand the cellular processes contributing to the progression of hepatic fibrosis. Interestingly in this perspective, a common feature of fibrotic disease of various organs is the activation of the coagulation cascade and hepatic fibrosis is also accompanied by a local hypercoagulable state. Activated blood coagulation factors directly target liver cells by activating protease-activated receptors (PAR) thereby inducing a plethora of cellular responses like (among others) proliferation, migration and extracellular matrix production. Coagulation factor driven PAR activation thus establishes a potential link between activation of the coagulation cascade and the progression of fibrosis. The current review focuses on blood coagulation factor Xa and summarizes the variety of cellular functions induced by factor Xa-driven PAR-2 activation and the subsequent consequences for tissue repair and hepatic fibrosis.

The inhibitory effect of Gynostemma pentaphyllum on MCP-1 and type I procollagen expression in rat hepatic stellate cells

AIM OF THE STUDY

Gynostemma pentaphyllum is a popular folk medicine that has been used for treatment of hepatitis in Asia. Our previous study demonstrates that Gynostemma pentaphyllum n-butanol extract inhibits the onset and improves the recovery of CCl(4)-induced liver fibrogenesis in rats and inhibits PDGF-induced rat hepatic stellate cells (HSCs) proliferation. In this study, the effect of Gynostemma pentaphyllum extract on cytokines and type I procollagen expression was analyzed.

MATERIALS AND METHODS

Rat HSCs were treated with PDGF, Gynostemma pentaphyllum n-butanol extract, RP-18-Gyp fraction, rapamycin or vehicle. Rat cytokine antibody array chip or ELISA kit was used for cytokines detection. Intracellular protein expression was detected by Western blotting, mRNA expression was analyzed by RT-PCR.

RESULTS

RP-18-Gyp fraction is the more purified gypenosides fraction from Gynostemma pentaphyllum n-butanol extract. In cell proliferation, the inhibitory effect of 200 microg/ml RP-18-Gyp fraction is similar to 500 microg/ml Gynostemma pentaphyllum n-butanol extract. Furthermore, both of them have the ability of decreasing monocyte chemoattractant protein-1 (MCP-1) mRNA expression and protein release and inhibiting type I procollagen protein expression.

CONCLUSIONS

Both of Gynostemma pentaphyllum n-butanol extract and its more purified RP-18-Gyp fraction have the biological activities in the inhibition of cell proliferation, MCP-1 release and type I procollagen expression in rat HSCs. These data could provide the evidence to support for the traditional use of Gynostemma pentaphyllum in hepatitis.

Therapeutic targeting of the PDGF and TGF-beta-signaling pathways in hepatic stellate cells by PTK787/ZK22258

Stimulation of hepatic stellate cells (HSCs) by platelet-derived growth factor (PDGF) and transforming growth factor-beta1 (TGF-beta1) is an essential pathway of proliferation and fibrogenesis, respectively, in liver fibrosis. We provide evidence that PTK787/ZK222584 (PTK/ZK), a potent tyrosine kinase inhibitor that blocks vascular endothelial growth factor receptor (VEGFR), significantly inhibits PDGF receptor expression, as well as PDGF-simulated HSC proliferation, migration and phosphorylation of ERK1/2, Akt and p70S6 kinase. Interestingly, PTK/ZK also antagonizes the TGF-beta1-induced expression of VEGF and VEGFR1. Furthermore, PTK/ZK downregulates TGF-beta receptor expression, which is associated with reduced Akt, ERK and p38MAPK phosphorylation. Furthermore, PDGF-induced TGF-beta1 expression is inhibited by PTK/ZK. These findings provide evidence that PTK/ZK targets multiple essential pathways of stellate cell activation that provoke proliferation and fibrogenesis. Our study underscores the potential use of PTK/ZK as an antifibrotic drug in chronic liver disease.

Advances in research on the signaling pathways involved in activation and phenotypic transformation of hepatic stellate cells and their inhibitors

Liver fibrosis is a progressive pathologic process that involves deposition of excess extracellular matrix leading to distorted architecture and culminating in cirrhosis. It is believed that activation and phenotypic transformation of hepatic stellate cells (HSCs) play a central role in the development and resolution of liver fibrosis. Many cytokines and related signaling pathways are involved in the phenotypic transformation and proliferation of HSCs. In recent years, great advances have been made in the study of these signaling pathways and their specific inhibitors, thereby providing a new avenue for clinical therapy of liver fibrosis. However, as the mechanisms underlying the roles of these signaling pathways are very complicated, further intensive studies are still essential. In this article, we will review the advances in research on the signaling pathways involved in activation and phenotypic transformation of hepatic stellate cells and their inhibitors.

Hepatic stellate cells express functional CXCR4: role in stromal cell-derived factor-1alpha-mediated stellate cell activation

Chemokine interactions with their receptors have been implicated in hepatic stellate cell (HSC) activation. The hepatic expression of CXCR4 messenger RNA is increased in hepatitis C cirrhotic livers and plasma levels of its endogenous ligand, stromal cell-derived factor-1alpha (SDF-1alpha), correlate with increased fibrosis in these patients. The expression of CXCR4 by HSCs has not been reported. We therefore examined whether HSCs express CXCR4 in vivo and in vitro and explored whether SDF-1alpha/CXCR4 receptor engagement promotes HSC activation, fibrogenesis, and proliferation. The hepatic protein expression of both CXCR4 and SDF-1alpha is increased in hepatitis C cirrhotic livers and immunoflourescent and immunohistochemical staining confirms that HSCs express CXCR4 in vivo. Immortalized human stellate cells as well as primary human HSCs express CXCR4, and cell surface receptor expression increases with progressive culture-induced activation. Treatment of stellate cells with recombinant SDF-1alpha increases expression of alpha-smooth muscle actin and collagen I and stimulates a dose-dependent increase in HSC proliferation. Inhibitor studies suggest that SDF-1alpha/CXCR4-dependent extracellular signal-regulated kinase 1/2 and Akt phosphorylation mediate effects on collagen I expression and stellate cell proliferation.

CONCLUSION

HSCs express CXCR4 receptor in vivo and in vitro. CXCR4 receptor activation by SDF-1alpha is profibrogenic through its effects on HSC activation, fibrogenesis, and proliferation. Extracellular signal-regulated kinase 1/2 and phosphoinositide 3-kinase pathways mediate SDF-1alpha-induced effects on HSC expression of collagen I and proliferation. The availability of small molecule inhibitors of CXCR4 make this receptor an appealing target for antifibrotic approaches.

Involvement of 90-kuD ribosomal S6 kinase in collagen type I expression in rat hepatic fibrosis

AIM: To investigate the relationship between 90-kuD ribosomal S6 kinase (p90RSK) and collagen type I expression during the development of hepatic fibrosis in vivo and in vitro.

METHODS: Rat hepatic fibrosis was induced by intraperitoneal injection of dimethylnitrosamine. The protein expression and cell location of p90RSK and their relationship with collagen type I were determined by co-immunofluoresence and confocal microscopy. Subsequently, RNAi strategy was employed to silence p90RSK mRNA expression in HSC-T6, an activated hepatic stellate cell (HSC) line. The expression of collagen type I in HSC-T6 cells was assessed by Western blotting and real-time polymerase chain reaction. Furthermore, HSCs were transfected with expression vectors or RNAi constructs of p90RSK to increase or decrease the p90RSK expression, then collagen type I promoter activity in the transfected HSCs was examined by reporter assay. Lastly HSC-T6 cells transfected with p90RSK siRNA was treated with or without platelet-derived growth factor (PDGF)-BB at a final concentration of 20 &mgr;g/L and the cell growth was determined by MTS conversion.

RESULTS: In fibrotic liver tissues, p90RSK was over-expressed in activated HSCs and had a significant positive correlation with collagen type I levels. In HSC-T6 cells transfected with RNAi targeted to p90RSK, the expression of collagen type I was down-regulated (61.8% in mRNA, P < 0.01, 89.1% in protein, P < 0.01). However, collagen type I promoter activity was not increased with over-expression of p90RSK and not decreased with low expression either, compared with controls in the same cell line (P = 0.076). Furthermore, p90RSK siRNA exerted the inhibition of HSC proliferation, and also abolished the effect of PDGF on the HSC proliferation.

CONCLUSION: p90RSK is over-expressed in activated HSCs and involved in regulating the abnormal expression of collagen type I through initiating the proliferation of HSCs.

Suppressive effect of orthovanadate on hepatic stellate cell activation and liver fibrosis in rats

Orthovanadate (OV), an inhibitor of protein tyrosine phosphatases, affects various biological processes in a cell-type-specific manner. In this study, we investigated the effect of OV on hepatic stellate cells (HSCs). When primary rat HSCs were cultured in the presence of 10% serum, they spontaneously lost characteristic stellate morphology, proliferated, and were transformed into an activated state with the formation of abundant stress fibers and increased expression of both alpha-smooth muscle actin and collagen type I mRNA. OV treatment inhibited proliferation and activation of HSCs and partially reversed the phenotype of activated HSCs. Among the signaling molecules investigated, phosphorylation of the Src protein at tyrosine 416 was the most striking in OV-treated HSCs. Treatment of cells with Src family inhibitors partially abrogated the effects of OV. Furthermore, transfection of v-Src into activated HSCs induced a stellate morphology similar to that in the quiescent state. We then examined whether OV could effectively suppress HSC activation in vivo after liver injury induced by either carbon tetrachloride or dimethylnitrosamine. OV significantly reduced the appearance of alpha-smooth muscle actin-positive cells and decreased collagen deposition, concomitant with an improvement in liver function. Our study showed for the first time that OV was able to suppress the activation of HSCs, possibly through the modulation of Src activity, and attenuated fibrosis after chronic liver injury.

Intracellular calcium signals regulate growth of hepatic stellate cells via specific effects on cell cycle progression

Hepatic stellate cells (HSC) are important mediators of liver fibrosis. Hormones linked to downstream intracellular Ca(2+) signals upregulate HSC proliferation, but the mechanisms by which this occurs are unknown. Nuclear and cytosolic Ca(2+) signals may have distinct effects on cell proliferation, so we expressed plasmid and adenoviral constructs containing the Ca(2+) chelator parvalbumin (PV) linked to either a nuclear localization sequence (NLS) or a nuclear export sequence (NES) to block Ca(2+) signals in distinct compartments within LX-2 immortalized human HSC and primary rat HSC. PV-NLS and PV-NES constructs each targeted to the appropriate intracellular compartment and blocked Ca(2+) signals only within that compartment. PV-NLS and PV-NES constructs inhibited HSC growth. Furthermore, blockade of nuclear or cytosolic Ca(2+) signals arrested growth at the G2/mitosis (G2/M) cell-cycle interface and prevented the onset of mitosis. Blockade of nuclear or cytosolic Ca(2+) signals downregulated phosphorylation of the G2/M checkpoint phosphatase Cdc25C. Inhibition of calmodulin kinase II (CaMK II) had identical effects on LX-2 growth and Cdc25C phosphorylation. We propose that nuclear and cytosolic Ca(2+) are critical signals that regulate HSC growth at the G2/M checkpoint via CaMK II-mediated regulation of Cdc25C phosphorylation. These data provide a new logical target for pharmacological therapy directed against progression of liver fibrosis.

The tumor suppressor protein PTEN inhibits rat hepatic stellate cell activation

BACKGROUND

Following a fibrogenic stimulus, the hepatic stellate cell (HSC) transforms from a quiescent to an activated cell type associated with increased proliferation, collagen and smooth muscle alpha-actin (alphaSMA) expression. Phosphatase and Tensin Homolog Deleted on Chromosome Ten (PTEN), a tumor suppressor phosphatase, has been shown to play a role in several nonmalignant diseases. Here, we investigated the role of PTEN during HSC activation.

METHODS

Rat HSCs 2 days after isolation were transduced with adenoviruses expressing either the wild-type (WT) or a dominant negative form of PTEN, and culture-associated activation of HSCs, including morphological changes, expression of alphaSMA and alpha1(I) collagen, and cell proliferation, were evaluated. Apoptosis of HSCs was detected by measuring activity of caspase 3/7. Phosphorylation status of Akt, p70(S6K), and Erk was detected by Western blotting.

RESULTS

Overexpression of WT-PTEN inhibited phenotypic changes were associated with HSC activation, including morphological changes, expression of alphaSMA and alpha1(I) collagen, and HSC proliferation, including cyclin D1 expression. WT-PTEN expression also induced apoptosis in HSCs with increased caspase 3/7 activity. Expression of WT-PTEN also caused decreased activation of Akt, p70(S6K), and Erk signaling pathways.

CONCLUSIONS

Taken together, these findings show that PTEN represents an important negative regulator for transactivation of HSCs. This may have important implications for the design of therapeutic strategies to prevent the progression of liver fibrosis.

Oxidative and nitrosative stress and fibrogenic response

Uncontrolled production of collagen I is the main feature of liver fibrosis. Following a fibrogenic stimulus such as alcohol, hepatic stellate cells (HSC) transform into an activated collagen-producing cell. In alcoholic liver disease, numerous changes in gene expression are associated with HSC activation, including the induction of several intracellular signaling cascades, which help maintain the activated phenotype and control the fibrogenic and proliferative state of the cell. Detailed analyses for understanding the molecular basis of the collagen I gene regulation have revealed a complex process involving reactive oxygen species (ROS) as key mediators. Less is known, however, about the contribution of reactive nitrogen species (RNS). In addition, a series of cytokines, growth factors, and chemokines, which activate extracellular matrix (ECM)-producing cells through paracrine and autocrine loops, contribute to the fibrogenic response.

RNA interference targeting the platelet-derived growth factor receptor beta subunit ameliorates experimental hepatic fibrosis in rats

BACKGROUND/AIMS

Platelet-derived growth factor (PDGF) is the strongest stimulator of the proliferation of hepatic stellate cells (HSCs). PDGF receptor beta subunit (PDGFR-beta) is acquired on HSCs proliferation induced by PDGF. In this study, we aim to investigate the effect of PDGFR-beta small interference RNA (siRNA) on experimental hepatic fibrosis.

METHODS

We constructed a PDGFR-beta siRNA expression plasmid and investigated its effect on the activation of HSCs. Bromodeoxyuridine incorporation was performed to investigate the effect of PDGFR-beta siRNA on HSCs proliferation. A hydrodynamics-based transfection method was used to deliver PDGFR-beta siRNA to rats with hepatic fibrosis. The distribution of transgenes in the liver was observed by immunofluorescence. The antifibrogenic effect of PDGFR-beta siRNA was investigated pathologically.

RESULTS

Platelet-derived growth factor receptor-beta subunit siRNA could significantly downregulate PDGFR-beta expression, suppress HSCs activation, block the mitogen-activated protein kinase signalling pathway and inhibit HSCs proliferation in vitro. PDGFR-beta siRNA expression plasmid could be delivered into activated HSCs by the hydrodynamics-based transfection method, and remarkably improve the liver function of the rat model induced by dimethylnitrosamine and bile duct ligation. Furthermore, the progression of fibrosis in the liver was significantly suppressed by PDGFR-beta siRNA in both animal models.

CONCLUSIONS

Platelet-derived growth factor receptor-beta subunit siRNA may be presented as an effective antifibrogenic gene therapeutic method for hepatic fibrosis.

Cytokines and renin-angiotensin system signaling in hepatic fibrosis

Hepatic fibrosis is the result of a complex interplay between resident hepatic cells, infiltrating inflammatory cells, and a number of locally acting peptides called cytokines. Key mediators include transforming growth factor b1, vasoactive substances, adipokines, inflammatory cytokines and chemokines. Angiotensin II, the main effector of the renin-angiotensin system, is a true cytokine that plays a major role in liver fibrosis. Angiotensin II is locally synthesized in the injured liver and induces profibrogenic actions in hepatic stellate cells. Drugs blocking the renin-angiotensin system are promising antifibrotic agents. There are multiple signal transduction pathways involved in cytokine signaling. Drugs interfering intracellular pathways involved in increased collagen production are potential therapies for liver fibrosis.

The epidermal growth factor receptor ligand amphiregulin participates in the development of mouse liver fibrosis

The hepatic wound-healing response to chronic noxious stimuli may lead to liver fibrosis, a condition characterized by excessive deposition of extracellular matrix. Fibrogenic cells, including hepatic stellate cells and myofibroblasts, are activated in response to a variety of cytokines, growth factors, and inflammatory mediators. The involvement of members of the epidermal growth factor family in this process has been suggested. Amphiregulin (AR) is an epidermal growth factor receptor (EGFR) ligand specifically induced upon liver injury. Here, we have addressed the in vivo role of AR in experimental liver fibrosis. To this end, liver fibrosis was induced in AR+/+ and AR-/- mice by chronic CCl(4) administration. Histological and molecular markers of hepatic fibrogenesis were measured. Additionally, the response of cultured human and mouse liver fibrogenic cells to AR was evaluated. We observed that AR was expressed in isolated Kupffer cells and liver fibrogenic cells in response to inflamatory stimuli and platelet-derived growth factor, respectively. We demonstrate that the expression of alpha-smooth muscle actin and collagen deposition were markedly reduced in AR-/- mice compared to AR+/+ animals. AR-/- mice also showed reduced expression of tissue inhibitor of metalloproteinases-1 and connective tissue growth factor, two genes that responded to AR treatment in cultured fibrogenic cells. AR also stimulated cell proliferation and exerted a potent antiapoptotic effect on isolated fibrogenic cells.

CONCLUSION

These results indicate that among the different EGFR ligands, AR plays a specific role in liver fibrosis. AR may contribute to the expression of fibrogenic mediators, as well as to the growth and survival of fibrogenic cells. Additionally, our data lend further support to the role of the EGFR system in hepatic fibrogenesis.

Hepatic fibrogenesis in response to chronic liver injury: novel insights on the role of cell-to-cell interaction and transition

Hepatic fibrosis represents the wound-healing response process of the liver to chronic injury, independently from aetiology. Advanced liver fibrosis results in cirrhosis that can lead to liver failure, portal hypertension and hepatocellular carcinoma. Currently, no effective therapies are available for hepatic fibrosis. After the definition of hepatic stellate cells (HSCs) as the main liver extracellular matrix-producing cells in the 1980s, the subsequent decade was dedicated to determine the role of specific cytokines and growth factors. Fibrotic progression of chronic liver diseases can be nowadays considered as a dynamic and highly integrated process of cellular response to chronic liver injury. The present review is dedicated to the novel mechanisms of cellular response to chronic liver injury leading to hepatic myofibroblasts' activation. The understanding of the cellular and molecular pathways regulating their function is crucial to counteract therapeutically the organ dysfunction caused by myofibroblasts' activation.

Targeted inhibition of platelet-derived growth factor receptor-beta subunit in hepatic stellate cells ameliorates hepatic fibrosis in rats

The activation of hepatic stellate cells (HSCs) is the key event of the pathogenesis of hepatic fibrosis. Platelet-derived growth factor (PDGF) is the most potent mitogen for HSCs, and PDGF receptor-beta subunit (PDGFR-beta) is required for the proliferation of HSCs induced by PDGF. In this study, a high gene-silencing-efficacy PDGFR-beta small interference RNA (siRNA) was synthesized that could suppress the PDGFR-beta expression and inhibit the activation and proliferation but could not induce the apoptosis of HSCs in vitro. To avoid the side effect of nonspecific interference of PDGFR-beta, we constructed an HSCs-specific short hairpin RNA (shRNA) expression plasmid in which PDGFR-beta shRNA was driven by a glial fibrillary acidic protein (GFAP) promoter. The double-staining immunofluorescence examination indicated that GFAP promoter could target the transgene expression into HSCs in carbon tetrachloride induced acute injured rat's liver and bile duct ligation (BDL)-induced chronic injured rat's liver. Furthermore, HSCs-specific PDGFR-beta shRNA could relieve liver injury and hepatic fibrosis in the rat's model induced by BDL. This study demonstrates that PDGFR-beta siRNA may be presented as an antifibrogenic agent. The application of HSCs-specific RNA interference induced by the GFAP promoter might supply a new powerful tool for cell-specific gene therapy of hepatic fibrogenesis.

The molecular mechanism of gypenosides-induced G1 growth arrest of rat hepatic stellate cells

AIM OF THE STUDY

Gypenosides, the saponins extract derived from Gynostemma pentaphyllum Makino, have been used for treating hepatitis and cancer in Asia. Our previous study demonstrates that gypenosides inhibit the onset and improve the recovery of liver fibrosis induced by CCl4 in rats. In this study, we used the isolated rat hepatic stellate cells (HSCs) as a model to study the cellular mechanism of gypenosides-inhibited liver fibrosis.

MATERIALS AND METHODS

Rat HSCs was treated with PDGF, gypenosides or vehicle. Cell viability was assessed by trypan blue staining. Apoptosis and cell cycle were evaluated by flow cytometry. The activation or inhibition of signal molecules was detected by Western blotting.

RESULTS

Our results showed that 500 microg/ml gypenosides decreased PDGF-induced rat HSCs numbers (8750+/-2629 versus 103,000+/-6683, p<0.001, 95% confidence interval) and arrested cells at the G1 phase without the presence of sub-G1 fraction. Analysis of PDGF-induced proliferative molecules including phosphorylation of Akt and p70 S6K, gypenosides inhibited the activation of this signal pathway. Furthermore, gypenosides down-regulated the protein expression of cell cycle G1-specific cyclin D1 and D3.

CONCLUSIONS

Gypenosides inhibited PDGF-induced HSCs proliferation by inhibiting the signal pathway of PDGF-Akt-p70 S6K and down-regulation of cyclin D1 and D3 expression.

Suppressive effect of leflunomide on rat hepatic stellate cell proliferation involves on PDGF-BB-elicited activation of three mitogen-activated protein kinases

In this manuscript, we demonstrated that following stimulus by Kupffer cell-conditioned medium (KCM) and PDGF-BB, hepatic stellate cells (HSCs) showed significant increases in DNA synthesis and PDGFR-beta expression. Furthermore, phosphorylation of PDGFR-beta and three major members of the mitogen-activated protein kinase (MAPK) family were also significantly increased. Studies with respective neutralizing antibodies against released cytokines in conditioned medium demonstrated that PDGF-BB played an essential role in this complex activation process. Administration of A771726, leflunomide's metabolite, markedly blunted these effects. However, the combination of A771726 with any of the three MAPK inhibitors potentiated this inhibitory effect and showed completely blockage on PDGFR-beta expression and phosphorylation. Collectively, these data demonstrate that leflunomide inhibits KCM-mediated HSC proliferation via PDGFR-beta phosphorylation and the subsequent activation of the MAPK pathway. Accordingly, targeted intervention against the PDGF-BB isoform may also offer a promising therapeutic approach to liver fibrosis.

Adenosine monophosphate-activated protein kinase modulates the activated phenotype of hepatic stellate cells

Adiponectin limits the development of liver fibrosis and activates adenosine monophosphate-activated protein kinase (AMPK). AMPK is a sensor of the cellular energy status, but its possible modulation of the fibrogenic properties of hepatic stellate cells (HSCs) has not been established. In this study, we investigated the role of AMPK activation in the biology of activated human HSCs. A time-dependent activation of AMPK was observed in response to a number of stimuli, including globular adiponectin, 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), or metformin. All these compounds significantly inhibited platelet-derived growth factor (PDGF)-stimulated proliferation and migration of human HSCs and reduced the secretion of monocyte chemoattractant protein-1. In addition, AICAR limited the secretion of type I procollagen. Knockdown of AMPK by gene silencing increased the mitogenic effects of PDGF, confirming the negative modulation exerted by this pathway on HSCs. AMPK activation did not reduce PDGF-dependent activation of extracellular signal-regulated kinase (ERK) or Akt at early time points, whereas a marked inhibition was observed 24 hours after addition of PDGF, reflecting a block in cell cycle progression. In contrast, AICAR blocked short-term phosphorylation of ribosomal S6 kinase (p70(S6K)) and 4E binding protein-1 (4EBP1), 2 downstream effectors of the mammalian target of rapamycin (mTOR) pathway, by PDGF. The ability of interleukin-a (IL-1) to activate nuclear factor kappa B (NF-kappaB) was also reduced by AICAR.

CONCLUSION

Activation of AMPK negatively modulates the activated phenotype of HSCs.

Protease-activated receptor 1 knockout reduces experimentally induced liver fibrosis

Thrombin inhibition protects against liver fibrosis. However, it is not known whether the thrombin profibrogenic effect is due to effects on blood coagulation or to signaling via protease-activated receptors (PARs). We took advantage of the lack of blood coagulation defects in PAR-1-knockout mice. Acute carbon tetrachloride (CCl(4)) toxicity was similar in wild-type (WT), PAR-1(-/-), and PAR-1(+/-) mice as judged by aminotransferase levels, area of liver necrosis, and liver peroxidation measured by Fourier-transformed infrared spectroscopy. Fifteen mice/group received CCl(4) or its solvent for 6 wk (300 microl/kg, 3 times a week). Fibrosis area was increased 10-fold by CCl(4) treatment in WT mice. PAR-1 deficiency protected against fibrosis, with 36% and 56% decrease in PAR-1(+/-) and PAR-1(-/-) mice, respectively (P < 0.001). Similar results were obtained for area of activated fibrogenic cells (64% and 79% decrease in PAR-1(+/-) and PAR-1(-/-) mice, respectively, P < 0.001). These findings were corroborated by measurements of type I collagen, matrix metalloproteinase-2, and PDGF-beta receptor mRNA levels. There was also a significant decrease in T lymphocyte infiltration in PAR-1-deficient mice. Altogether, these results suggest that thrombin profibrogenic effects are independent of effects on blood coagulation and are instead due to direct effects on fibrogenic cells and possibly on T lymphocytes.

NIM811, a nonimmunosuppressive cyclosporine analogue, suppresses collagen production and enhances collagenase activity in hepatic stellate cells

BACKGROUND/AIMS

A recent decrease in patient survival has been reported among hepatitis C virus (HCV)-infected liver transplant recipients and this may be attributable to progression of fibrosis. We reported previously that cyclosporine suppressed the proliferation of, and collagen production in, hepatic stellate cells (HSCs). Here, we investigated the effects of NIM811, a cyclosporine analogue, on cell growth, collagen production and collagenase activity in HSCs.

METHODS

Rat HSCs and human HSC-derived TWNT-4 cells were cultured for the study. The expression of collagen, matrix metalloproteinase 1 (MMP-1) and tissue inhibitor of metalloproteinase-1 (TIMP-1) and collagenase activity was evaluated. Cell proliferation and apoptosis were measured. Phosphorylation of mitogen-activated protein kinases (MAPKs), Smad2 and Smad3 was evaluated. The expression of the tumour growth factor-beta (TGF-beta)-receptor and Smad7 genes was also evaluated.

RESULTS

NIM811, as well as cyclosporine, suppressed the transcription and synthesis of collagen and stimulated the production of MMP-1 with a concomitant enhancement of collagenase activity, although it did not change the expression of TIMP-1. NIM811 inhibited proliferation without induction of apoptosis. In the MAPKs and TGF-beta signalling pathways, NIM811 enhanced the phosphorylation of JNK and p38, but not extracellular signal-regulated kinases 1 and 2, and suppressed the phosphorylation of Smad2 and Smad3, accompanied by increased Smad7 transcription and decreased TGF-beta-receptor transcription.

CONCLUSION

These findings demonstrate that NIM811 not only suppresses collagen production and proliferation but also increases collagenase activity. These effects are accompanied by inhibition of TGF-beta signalling pathways.

Hyperglycemia and hyperinsulinemia have additive effects on activation and proliferation of pancreatic stellate cells: possible explanation of islet-specific fibrosis in type 2 diabetes mellitus

Pancreatic islet fibrosis observed in Type 2 diabetes is one of the major factors leading to progressive beta-cell loss and dysfunction. Despite its importance, the mechanism of islet-restricted fibrogenesis associated with pancreatic stellate cell (PSC) activation and proliferation remains to be defined. Therefore, we studied whether the islet-specific environment represented by hyperglycemia and hyperinsulinemia had additive effects on the activation and proliferation of cultured rat PSCs. Cells were stimulated to activate and proliferate with glucose and insulin, either individually or concomitantly. Both stimuli promoted PSC proliferation and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation independently, but an additive effect was also demonstrated. Blockade of ERK signaling by the mitogen-activated protein kinase kinase (MEK) inhibitor, U0126, suppressed both glucose- and insulin-induced ERK 1/2 phosphorylation and PSC proliferation. Glucose and insulin-induced ERK 1/2 phosphorylation also stimulated connective tissue growth factor gene expression. Thus, hyperglycemia and hyperinsulinemia are two crucial mitogenic factors that activate and proliferate PSCs, and the presence of both states will amplify this response.

Effects of extracellular signal-regulated kinase on rat cultured hepatic stellate cells stimulated by acetaldehyde

OBJECTIVE

To investigate the effects of PD98059 on the cell cycle, cell proliferation, the secretion of type I collagen and expression of transforming growth factor-beta-1 mRNA in rat hepatic stellate cells stimulated by acetaldehyde.

METHODS

Rat hepatic stellate cells stimulated by acetaldehyde were incubated with different concentrations of PD98059. The cell cycle was analyzed by flow cytometry. Cell proliferation was assessed by methyl thiazolyl tetrazolium colorimetric assay. The mRNA expression of transforming growth factor-beta-1 was examined by reverse transcriptase polymerase chain reaction. Type I collagen of the culture medium was detected by enzyme-linked immunoadsorbent assay.

RESULTS

Twenty, 50 and 100 micromol/L PD98059 could significantly inhibit the proliferation and provoke a G0/G1-phase arrest of hepatic stellate cells stimulated by acetaldehyde in a dose-dependent manner. The secretion of type I collagen and transforming growth factor-beta-1 mRNA expression of acetaldehyde-induced hepatic stellate cells were markedly inhibited by 50 and 100 micromol/L PD98059, respectively.

CONCLUSION

Extracellular signal-regulated kinase signal transduction pathway could regulate cell proliferation, the secretion of type I collagen and transforming growth factor-beta-1 mRNA expression of rat hepatic stellate cells stimulated by acetaldehyde. This is most likely related to its regulative effect on the cell cycle.

Glucagon-like peptide-1 and its receptor agonist exendin-4 modulate cholangiocyte adaptive response to cholestasis

BACKGROUND & AIMS

Cholangiopathies are characterized by progressive dysregulation of the balance between proliferation and death of cholangiocytes. In the course of cholestasis, cholangiocytes undergo a neuroendocrine transdifferentiation and their biology is regulated by neuroendocrine hormones. Glucagon-like peptide-1 (GLP-1), secreted by neuroendocrine cells, sustains beta-cell survival in experimental diabetes and induces the neuroendocrine transdifferentiation of pancreatic ductal cells. GLP-1 receptor (GLP-1R) selective agonist exendin-4 is used in humans as a novel therapeutic tool for diabetes. The aim of this study was to define if GLP-1 modulates cholangiocyte biologic response to cholestasis.

METHODS

Expression of GLP-1R in cholangiocytes was determined. Effects on cholangiocyte proliferation of the in vitro and in vivo exposure to GLP-1 or exendin-4, together with the intracellular signals, were then studied. Synthesis of GLP-1 by cholangiocytes and the effects of GLP-1R blockage on their growth were also determined.

RESULTS

Cholangiocytes express the GLP-1 receptor, which is up-regulated in the course of cholestasis. GLP-1 and exendin-4 increase cholangiocyte growth both in vitro and in vivo. The GLP-1R signal is mediated by the phosphatidyl-inositol-3-kinase, cAMP/Protein Kinase A, and Ca(2+)-CamKIIalpha but not by the ERK1/2 and PKCalpha pathways. Proliferating cholangiocytes synthesize GLP-1: neutralization of its action by GLP-1R antagonist blunts cholangiocyte response to cholestasis.

CONCLUSIONS

GLP-1 is required for the cholangiocyte adaptive response to cholestasis. Cholangiocytes are susceptible to the activation of GLP-1R and respond with increased proliferation and functional activity. Exendin-4 availability for employment in humans and these data may open novel perspectives for the medical treatment of cholangiopathies.

Molecular mechanisms of hepatic fibrogenesis

Liver fibrosis, a wound-healing response to a variety of chronic stimuli, is characterized by excessive deposition of extracellular matrix (ECM) proteins, of which type I collagen predominates. This alters the structure of the liver leading to organ dysfunction. The activated hepatic stellate cell (HSC) is primarily responsible for excess collagen deposition during liver fibrosis. Two important aspects are involved in mediating the fibrogenic response: first the HSC becomes directly fibrogenic by synthesizing ECM proteins; second, the activated HSC proliferates, effectively amplifying the fibrogenic response. Although the precise mechanisms responsible for HSC activation remain elusive, substantial insight is being gained into the molecular mechanisms responsible for ECM production and cell proliferation in the HSC. The activated HSC becomes responsive to both proliferative (platelet-derived growth factor) and fibrogenic (transforming growth factor-beta[TGF-beta]) cytokines. It is becoming clear that these cytokines activate both mitogen-activated protein kinase (MAPK) signaling, involving p38, and focal adhesion kinase-phosphatidylinositol 3-kinase-Akt-p70 S6 kinase (FAK-PI3K-Akt-p70(S6K)) signaling cascades. Together, these regulate the proliferative response, activating cell cycle progression as well as collagen gene expression. In addition, signaling by both TGF-beta, mediated by Smad proteins, and p38 MAPK influence collagen gene expression. Smad and p38 MAPK signaling have been found to independently and additively regulate alpha1(I) collagen gene expression by transcriptional activation while p38 MAPK, but not Smad signaling, increases alpha1(I) collagen mRNA stability, leading to increased synthesis and deposition of type I collagen. It is anticipated that by understanding the molecular mechanisms responsible for HSC proliferation and excess ECM production new therapeutic targets will be identified for the treatment of liver fibrosis.

Resistin as an intrahepatic cytokine: overexpression during chronic injury and induction of proinflammatory actions in hepatic stellate cells

Obesity and insulin resistance accelerate the progression of fibrosis during chronic liver disease. Resistin antagonizes insulin action in rodents, but its role in humans is still controversial. The aims of this study were to investigate resistin expression in human liver and to evaluate whether resistin may affect the biology of activated human hepatic stellate cells (HSCs), key modulators of hepatic fibrogenesis. Resistin gene expression was low in normal human liver but was increased in conditions of severe fibrosis. Up-regulation of resistin during chronic liver damage was confirmed by immunohistochemistry. In a group of patients with alcoholic hepatitis, resistin expression correlated with inflammation and fibrosis, suggesting a possible action on HSCs. Exposure of cultured HSCs to recombinant resistin resulted in increased expression of the proinflammatory chemokines monocyte chemoattractant protein-1 and interleukin-8, through activation of nuclear factor (NF)-kappaB. Resistin induced a rapid increase in intracellular calcium concentration, mainly through calcium release from intracellular inositol triphosphate-sensitive pools. The intracellular calcium chelator BAPTA-AM blocked resistin-induced NF-kappaB activation and monocyte chemoattractant protein-1 expression. In conclusion, this study shows a role for resistin as an intrahepatic cytokine exerting proinflammatory actions in HSCs, via a Ca2+/NF-kappaB-dependent pathway and suggests involvement of this adipokine in the pathophysiology of liver fibrosis.

Resistin as an intrahepatic cytokine: overexpression during chronic injury and induction of proinflammatory actions in hepatic stellate cells

Obesity and insulin resistance accelerate the progression of fibrosis during chronic liver disease. Resistin antagonizes insulin action in rodents, but its role in humans is still controversial. The aims of this study were to investigate resistin expression in human liver and to evaluate whether resistin may affect the biology of activated human hepatic stellate cells (HSCs), key modulators of hepatic fibrogenesis. Resistin gene expression was low in normal human liver but was increased in conditions of severe fibrosis. Up-regulation of resistin during chronic liver damage was confirmed by immunohistochemistry. In a group of patients with alcoholic hepatitis, resistin expression correlated with inflammation and fibrosis, suggesting a possible action on HSCs. Exposure of cultured HSCs to recombinant resistin resulted in increased expression of the proinflammatory chemokines monocyte chemoattractant protein-1 and interleukin-8, through activation of nuclear factor (NF)-kappaB. Resistin induced a rapid increase in intracellular calcium concentration, mainly through calcium release from intracellular inositol triphosphate-sensitive pools. The intracellular calcium chelator BAPTA-AM blocked resistin-induced NF-kappaB activation and monocyte chemoattractant protein-1 expression. In conclusion, this study shows a role for resistin as an intrahepatic cytokine exerting proinflammatory actions in HSCs, via a Ca2+/NF-kappaB-dependent pathway and suggests involvement of this adipokine in the pathophysiology of liver fibrosis.

Phospholipase D and extracellular signal-regulated kinase in hepatic stellate cells: effects of platelet-derived growth factor and extracellular nucleotides

We have previously provided evidence suggesting that phosphatidic acid, possibly derived from the hydrolysis of phosphatidylcholine by phospholipase D (PLD), is involved in platelet-derived growth factor (PDGF)-mediated increases in extracellular signal-regulated kinase (ERK) activity and DNA synthesis in rat hepatic stellate cells (HSC), the primary fibrogenic cells of the liver. A recent study has shown the presence of P2Y nucleotide receptors on HSC that are coupled to contraction and synthesis of the matrix component, alpha1-procollagen, leading to the suggestion that they may represent a new therapeutic target in the treatment of liver fibrosis. However, although extracellular nucleotides have been shown to stimulate both PLD and ERK, and to elicit proliferation of fibrogenic cells outside the liver, their effect on these parameters in HSC have not yet been investigated. PLD activity was determined by [3H]choline release and [3H]phosphatidylbutanol production, ERK activity by Western blotting, and DNA synthesis by [3H]thymidine incorporation. We report here, for the first time in HSC, that extracellular nucleotides stimulate PLD activity and a sustained activation of ERK. However, in contrast to PDGF, nucleotides had negligible effects on DNA synthesis. Moreover, the effects of PDGF and nucleotides on PLD and ERK were not additive, suggesting activation of the same PLD isoform and pool of ERK. The data demonstrate that nucleotide-stimulated PLD and ERK activities are not coupled to DNA synthesis in HSC. Instead, these responses may be linked to other phenotypic changes associated with activated HSC such as increases in contraction, motility, or extracellular matrix deposition.

2′,4′,6′-Tris(methoxymethoxy) chalcone attenuates hepatic stellate cell proliferation by a heme oxygenase-dependent pathway

Proliferation of hepatic stellate cells (HSCs) is central for the development of fibrosis during liver injury. We have shown previously that butein (3,4,2',4'-tetrahydroxychalcone) suppresses myofibroblastic differentiation of rat HSCs. Our aim in this study was to determine whether a new synthetic chalcone derivative, 2',4',6'-tris(methoxymethoxy) chalcone (TMMC) inhibits HSC proliferation induced by serum- or platelet-derived growth factor (PDGF). TMMC significantly inhibited growth factor-induced HSC proliferation. The inhibition of PDGF-induced proliferation by TMMC was associated with the phosphatidylinositol 3-kinase-Akt-p70(S6K) pathways. TMMC induced the expression of heme oxygenase 1 (HO-1) in HSCs. Using the chemical inhibitor tin protoporphyrin, we also found that the inhibitory action of TMMC on PDGF-induced proliferation is mediated by HO-1. Glutathione (GSH) depletion produced by TMMC activated extracellular signal-regulated kinase (ERK), which led to c-Fos expression and transactivation of activator protein 1 (AP-1) and HO-1 gene expression in the HSCs. These results demonstrate that TMMC preferentially activates ERK and that this activation leads to the transcriptional activation of AP-1 and consequently to HO-1 expression. HO-1 expression might be responsible for the antiproliferative effect of TMMC on HSCs.

Dynamic expression of extracellular signal-regulated kinase in rat liver tissue during hepatic fibrogenesis

AIM: To investigate whether extracellular signal-regulated kinase 1 (ERK₁) is activated and associated with hepatic stellate cell (HSC) proliferation in fibrotic rat liver tissue.

METHODS: Rat hepatic fibrosis was induced by bile duct ligation (BDL). Histopathological changes were evaluated by hematoxylin and eosin staining, and Masson’s trichrome method. ERK₁ mRNA in rat liver tissue was determined by reverse transcription-polymerase chain reaction, while the distribution of ERK₁ was assessed by immunohistochemistry. ERK₁ protein was detected by Western blotting analysis. The number of activated HSCs was quantified after alpha smooth muscle actin (α-SMA) staining.

RESULTS: With the development of hepatic fibrosis, the positive staining cells of α-SMA increased obviously, and mainly resided in the portal ducts. Fiber septa and perisinuses were accompanied with proliferating bile ducts. The positive staining areas of the rat livers in model groups 1-4 wk after ligation of common bile duct (12.88% ± 2.63%, 22.65% ± 2.16%, 27.45% ± 1.86%, 35.25% ± 2.34%, respectively) were significantly larger than those in the control group (5.88% ± 1.46%, P < 0.01). With the development of hepatic fibrosis, the positive cells of ERK₁ increased a lot, and were mainly distributed in portal ducts, fiber septa around the bile ducts, vascular endothelial cells and perisinusoidal cells. Western blotting analysis displayed that the expression of ERK₁ and ERK₂ protein was up-regulated during the model course, and its level was the highest 4 wk after operation, being 3.9-fold and 7.2-fold higher in fibrotic rat liver than in controls. ERK₁ mRNA was expressed in normal rat livers as well, which was up-regulated two days after BDL and reached the highest 4 wk after BDL. The expression of ERK₁ was positively correlated with α-SMA expression (r = 0.958，P < 0.05).

CONCLUSION: The expression of ERK₁ protein and mRNA is greatly increased in fibrotic rat liver tissues, which may play a key role in HSC proliferation and hepatic fibrogenesis.

Study on effects of extracts fromSalvia Miltiorrhiza andCurcuma Longa in inhibiting phosphorylated extracellular signal regulated kinase expression in rat’s hepatic stellate cells

OBJECTIVE

To study the effect of salvianolic acid B (SAB) and curcumin, the extracts of Salvia Miltiorrhiza and Curcuma Longa, on the proliferation and activation of hepatic stellate cell (HSC), and the extracellular signal regulated kinase (ERK) expression in it.

METHODS

Rat's HSC-T6 were cultured and treated by SAB or curcumin. The inhibitory effect on cell proliferation was determined by 3-(4, 5-dimthyl-2-2thiazoly)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) colorimetry, and the expression levels of alpha smooth actin (alpha-SMA), collagen type I, and ERK were determined by Western blot.

RESULTS

SAB and curcumin inhibited the proliferation and activation of rat's HSC-T6 in dose-dependent fashion and significantly reduced the expression level of alpha-SMA (P < 0.01). Curcumin significantly reduced the expression of collagen type I (P < 0.05). Both SAB and curcumin showed insignificant effect on the ERK expression level, but they could significantly reduce the level of phosphorylated-ERK expression, showing significant difference as compared with that in the control group (P < 0.01 and P < 0.05 respectively).

CONCLUSION

SAB and curcumin could significantly inhibit the proliferation, activation of HSC, and the production of type I collagen in HSC, the mechanism may be associated with their inhibition on ERK phosphorylation.

Inhibitory effects of Ligusticum chuanxiong on the proliferation of rat hepatic stellate cells

BACKGROUND AND AIMS

Platelet-derived growth factor (PDGF) is a very potent mitogen for hepatic stellate cells (HSC) in hepatic fibrogenesis. Ligusticum chuanxiong Hort. (LC), a traditional Chinese herb used for cerebrovascular diseases, has been shown to exert anti-inflammatory and free radical scavenging effects. The aims of the present study were to investigate the effects of LC extract on the proliferation-related biomarkers in a rat HSC cell line (HSC-T6) stimulated with PDGF.

METHODS

DNA synthesis via bromodeoxyuridine (BrdU) incorporation, cell cycle related proteins and apoptosis markers were determined to evaluate the inhibitory effects of LC.

RESULTS

The results revealed that LC extract (25-100 microg/mL) concentration-dependently decreased the PDGF-induced cell proliferation as well as alpha-smooth muscle actin expression in HSC. The inhibitory activity of LC on HSC was associated with: (i) inhibition of BrdU incorporation; (ii) induction of apoptosis with the activation of caspase-3, up-regulation of cell cycle inhibitory proteins p21 and p27, and down-regulation of cell cycle stimulatory proteins cyclins D1 and D2; and (iii) increased phosphorylation of mitogen-activated protein kinases (JNK). LC at the studied concentrations showed no direct cytotoxicity on primary hepatocytes.

CONCLUSION

The results suggest that LC significantly inhibited PDGF-activated HSC proliferation, possibly through apoptotic mechanisms and the potential of LC as an antifibrotic agent warrants further investigation.

Coordinate activation of intracellular signaling pathways by insulin-like growth factor-1 and platelet-derived growth factor in rat hepatic stellate cells

Proliferation of activated hepatic stellate cells (HSC) is an important event in the development of hepatic fibrosis. Insulin-like growth factor-1 (IGF-1) has been shown to be mitogenic for HSC, but the intracellular signaling pathways involved have not been fully characterized. Thus, the aims of the current study were to examine the roles of the extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3-K) and p70-S6 kinase (p70-S6-K) signaling pathways in IGF-1- and platelet-derived growth factor (PDGF)-induced mitogenic signaling of HSC and to examine the potential crosstalk between these pathways. Both IGF-1 and PDGF increased ERK, PI3-K and p70-S6-K activity. When evaluating potential crosstalk between these signaling pathways, we observed that PI3-K is required for p70-S6-K activation by IGF-1 and PDGF, and is partially responsible for PDGF-induced ERK activation. PDGF and IGF-1 also increased the levels of cyclin D1 and phospho-glycogen synthase kinase-3beta. Coordinate activation of ERK, PI3-K and p70-S6-K is important for perpetuating the activated state of HSC during fibrogenesis.

Endogenous opioids modulate the growth of the biliary tree in the course of cholestasis

BACKGROUND & AIMS

There is poor knowledge on the factors that modulate the growth of cholangiocytes, the epithelial cell target of cholangiopathies, which are diseases leading to progressive loss of bile ducts and liver failure. Endogenous opioids are known to modulate cell growth. In the course of cholestasis, the opioidergic system is hyperactive, and in cholangiocytes a higher expression of opioid peptide messenger RNA has been described. This study aimed to verify if such events affect the cholangiocyte proliferative response to cholestasis.

METHODS

The presence of the delta opioid receptor (OR), muOR, and kappaOR was evaluated. The effects on cholangiocyte proliferation of the in vitro and in vivo exposure to their selective agonists, together with the intracellular signals, were then studied. The effects of the OR antagonist naloxone on cell growth were also tested both in vivo and in vitro.

RESULTS

Cholangiocytes express all 3 receptors studied. deltaOR activation strongly diminished the proliferative and functional response of cholangiocytes to cholestasis, whereas muOR resulted in a slight increase in cell growth. The deltaOR signal is mediated by the IP3/CamKIIalpha/PKCalpha pathway, which inhibits the cAMP/PKA/ERK1/2/AKT cascade. In contrast, muOR activation stimulates the cAMP/PKA/ERK1/2/AKT cascade but does not affect the IP3/CamKIIalpha/PKCalpha pathway. The blockage of endogenous opioid peptides by naloxone further enhanced cholangiocyte growth both in vivo and in vitro.

CONCLUSIONS

The increase in opioid peptide synthesis in the course of cholestasis aims to limit the excessive growth of the biliary tree in the course of cholestasis by the interaction with the deltaOR expressed by cholangiocytes.

MEK hyperphosphorylation coincides with cell cycle shut down of cultured smooth muscle cells

Smooth muscle cells (SMCs) form the backbone of arteries and their proliferation hallmarks collateral vessel growth, a process termed arteriogenesis, as well as pathogenic responses such as restenosis. Since signaling pathways in SMCs are the main targets for therapeutic interventions, we aimed to determine how and to what extent the activation of the ubiquitous MEK-ERK signaling pathway correlates with important in vivo phenomena such as dedifferentiation, nuclear activation and proliferation of SMCs. Specificity of this pathway was monitored using MEK inhibitors UO126 and PD98059 in platelet derived growth factor-AB (PDGF-AB)- and fibroblast growth factor-2 (FGF-2)-stimulated SMCs. PDGF-AB induced a rapid MEK activation followed by phosphorylation of the MEK substrates ERK1/2 while FGF-2 showed a less pronounced and delayed activation. Both growth factors triggered a marked phosphorylation of c-Myc and expression of Egr1. Pretreatment with MEK inhibitors suppressed the activation of the ERK cascade, abolished the down-regulation of desmin and led to cell cycle arrest. However, the reversibility of p27Kip1 down-regulation by UO126 was mainly observed after PDGF-AB stimulation, indicating MEK independent p27Kip1 down-regulation by FGF-2. Surprisingly, treatment of SMCs with UO126 or PD98059 increased the level of MEK phosphorylation in a dose dependent manner at serine residues 217/221 in the presence as well as in the absence of both growth factors. Our results strongly imply that depending on the environmental context phosphorylation of serines 217/221 serves as an "on" as well as an "off " switch.

Global gene expression profiling of dimethylnitrosamine-induced liver fibrosis: from pathological and biochemical data to microarray analysis

The development of hepatocellular carcinoma (HCC) is generally preceded by cirrhosis, which occurs at the end stage of fibrosis. This is a common and potentially lethal problem of chronic liver disease in Asia. The development of microarrays permits us to monitor transcriptomes on a genome-wide scale; this has dramatically speeded up a comprehensive understanding of the disease process. Here we used dimethylnitrosamine (DMN), a nongenotoxic hepatotoxin, to induce rat necroinflammatory and hepatic fibrosis. During the 6-week time course, histopathological, biochemical, and quantitative RT-PCR analyses confirmed the incidence of necroinflammatory and hepatic fibrosis in this established rat model system. Using the Affymetrix microarray chip, 256 differentially expressed genes were identified from the liver injury samples. Hierarchical clustering of gene expression using a gene ontology database allowed the identification of several stage-specific characters and functionally related clusters that encode proteins related to metabolism, cell growth/maintenance, and response to external challenge. Among these genes, we classified 44 potential necroinflammatory-related genes and 62 potential fibrosis-related markers or drug targets based on histopathological scores. We also compared the results with other data on well-known markers and various other microarray datasets that are available. In conclusion, we believe that the molecular picture of necroinflammatory and hepatic fibrosis from this study may provide novel biological insights into the development of early liver damage molecular classifiers than can be used for basic research and in clinical applications. A public accessible website is available at http://LiverFibrosis.nchc.org.tw:8080/LF.

Disruption of intermolecular disulfide bonds in PDGF-BB dimers by N-acetyl-L-cysteine does not prevent PDGF signaling in cultured hepatic stellate cells

Oxidative stress is important in the pathogenesis of liver fibrosis through its induction of hepatic stellate cell (HSC) proliferation and enhancement of collagen synthesis. Reactive oxygen species have been found to be essential second messengers in the signaling of both major fibrotic growth factors, platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta), in cultured HSC and liver fibrosis. The non-toxic aminothiol N-acetyl-L-cysteine (NAC) inhibits cellular activation and attenuates experimental fibrosis in liver. Prior reports show that NAC is capable of reducing the effects of TGF-beta in biological systems, in cultured endothelial cells, and HSC through its direct reducing activity upon TGF-beta molecules. We here analyzed the effects of NAC on PDGF integrity, receptor binding, and downstream signaling in culture-activated HSC. We found that NAC dose-dependently induces disintegration of PDGF in vitro. However, even high doses (>20mM) were not sufficient to prevent the phosphorylation of the PDGF receptor type beta, extracellular signal-regulated kinase, or protein kinase B (PKB/Akt). Therefore, we conclude that the PDGF monomer is still active. The described antifibrotic effects are therefore mainly attributable to the structural impairment of TGF-beta signaling components reported previously.

Mechanisms of liver fibrosis

Liver fibrosis represents a significant health problem worldwide of which no acceptable therapy exists. The most characteristic feature of liver fibrosis is excess deposition of type I collagen. A great deal of research has been performed to understand the molecular mechanisms responsible for the development of liver fibrosis. The activated hepatic stellate cell (HSC) is the primary cell type responsible for the excess production of collagen. Following a fibrogenic stimulus, HSCs change from a quiescent to an activated, collagen-producing cell. Numerous changes in gene expression are associated with HSC activation including the induction of several intracellular signaling cascades, which help maintain the activated phenotype and control the fibrogenic and proliferative state of the cell. Detailed analyses in understanding the molecular basis of collagen gene regulation have revealed a complex process offering the opportunity for multiple potential therapeutic strategies. However, further research is still needed to gain a better understanding of HSC activation and how this cell maintains its fibrogenic nature. In this review we describe many of the molecular events that occur following HSC activation and collagen gene regulation that contribute to the fibrogenic nature of these cells and provide a review of therapeutic strategies to treat this disease.

Effects of Yigankang and its separated recipe on expression of platelet-derived growth factor and its receptor in hepatic stellate cells

AIM: To investigate the effects of Yigankang and small compound of radix salviae militiorrhizae on protein and gene expression of platelet-derived growth factor-BB (PDGF-BB) and platelet-derived growth factor receptor-b (PDGFR-b) in hepatic stellate cells (HSCs).

METHODS: The purified extracts and pharmaceutical serum from normal and liver fibrosis rats of Yigankang and small compound of radix salviae militiorrhizae were prepared respectively. The expression of PDGF-BB mRNA and protein were detected by reverse transcription polymerase chain reaction (RT-PCR) and Western blot, and the expression of PDGFR-b mRNA and protein were measured by RT-PCR and flow cytometry.

RESULTS: The purified extracts of Yigankang and small compound of radix salviae militiorrhizae decreased PDGF-BB protein expression by 78.79% and 56.77% and its mRNA by 67.13% and 43.59% (P<0.01); lowered PDGFR-b protein expression by 11.61% and 6.79% and its mRNA by 54.85% and 30.51% respectively (P<0.01). Yigankang and small compound of radix salviae militiorrhizae pharmaceutical serum from normal rats decreased PDGF-BB protein expression by 68.05% and 49.95% and its mRNA by 55.87% and 38.66% (P<0.01); lowered PDGFR-b protein expression by 10.98% and 6.48% and its mRNA by 39.67% and 21.38% respectively (P<0.01). Yigankang and small formula of radix salviae militiorrhizae pharmaceutical serum from liver fibrosis rats reduced PDGF-BB protein expression by 81.93% and 67.31% and its mRNA by 72.68% and 55.49% (P<0.01); decreased PDGFR-b protein expression by 17.43% and 11.15% and its mRNA by 60.46% and 46.47% respectively (P<0.01). With respect to the inhibitory effects mentioned above, Yigankang had more evidently action than its separated recipe did, and the pharmaceutical serum from liver fibrosis rats was superior to that from normal rats.

CONCLUSION: The inhibitory effect of Yigankang and its separated recipe on PDGF-BB and PDGFR-b expression may be one of the main mechanisms of their antifibrotic action, and the whole recipe was superior to the separated recipe.

Growth inhibition and apoptosis in liver myofibroblasts promoted by hepatocyte growth factor leads to resolution from liver cirrhosis

Liver cirrhosis is characterized by hepatic dysfunction with extensive accumulation of fibrous tissue in the liver. In response to chronic hepatic injury, hepatic portal myofibroblasts and activated hepatic stellate cells (HSCs) play a role in liver fibrosis. Although administration or gene expression of hepatocyte growth factor (HGF) leads to improvement in hepatic fibrosis/cirrhosis, the related mechanisms are not fully understood. We investigated mechanisms involved in resolution from liver cirrhosis by HGF, focusing on growth regulation and apoptosis in portal myofibroblasts. Cultured rat HSCs could not proliferate, were withdrawn after passage, and were replaced by proliferating portal myofibroblasts during the passages. In quiescent HSCs, c-Met receptor expression was undetected whereas c-Met receptor expression was detected in activated HSCs and liver myofibroblasts expressing alpha-smooth muscle actin (alpha-SMA), suggesting that activated HSCs and portal myofibroblasts are targets of HGF. For cultured rat portal myofibroblasts, HGF counteracted phosphorylation of extracellular signal-regulated kinase (Erk) 1/2 and mitogenic stimulus induced by platelet-derived growth factor, induced c-jun N-terminal kinase (JNK) 1 phosphorylation, and promoted apoptotic cell death. In the dimethylnitrosamine rat model of liver cirrhosis, administration of HGF suppressed proliferation while promoting apoptosis of alpha-SMA-positive cells in the liver, events that were associated with reduced hepatic expressions of alpha-SMA and histological resolution from liver cirrhosis. Growth inhibition and enhanced apoptosis in portal myofibroblasts by HGF are newly identified mechanisms aiding resolution from liver fibrosis/cirrhosis by HGF.